Electrolytes are materials which are typically used to conduct an electric current in an electrochemical device. Ionic liquids are useful electrolytes because they contain high concentrations of mobile charged ions, and therefore may conduct substantial electric currents through movement of these charged ions.
It is advantageous for an ionic liquid electrolyte to (1) be stable in the liquid phase and have high conductivity at temperatures varying from room temperature down to -40.degree. C.; (2) be capable of dissolving large mole fractions of alkali metal salts; (3) be stable in the presence of alkali metals; (4) have a wide electrochemical window; and (5) enable reversible electrochemical deposition of alkali metals under charging conditions and stripping of alkali metals under discharging conditions in an electrochemical cell.
In order to obtain these desirable properties, many researchers have focused on certain organic cation-containing salts such as tetraalkyl ammonium tetrachloroaluminates or imidazolium tetrachloroaluminates because they are liquids at room temperature. Mixtures of certain of these compounds with alkali tetrachloroaluminates may be liquid at room temperature and exhibit some desirable properties such as high conductivity. However, these mixtures do not have practical application because they have organic cations which decompose in the presence of alkali metals.